Evolution requires the generation of genetic diversity (diversity in nucleic acid) followed by the selection of those nucleic acids which result in beneficial characteristics. Because the nucleic acid and the activity of the encoded gene product of an organism are physically linked (the nucleic acids being confined within the cells which they encode) multiple rounds of mutation and selection can result in the progressive survival of organisms with increasing fitness. Systems for rapid evolution of nucleic acids or proteins in vitro should mimic this process at the molecular level in that the nucleic acid and the activity of the encoded gene product must be linked and the activity of the gene product must be selectable.
Recent advances in molecular biology have allowed some molecules to be co-selected according to their properties along with the nucleic acids that encode them. The selected nucleic acids can subsequently be cloned for further analysis or use, or subjected to additional rounds of mutation and selection.
Common to these methods is the establishment of large libraries of nucleic acids. Molecules having the desired characteristics (activity) can be isolated through selection regimes that select for the desired activity of the encoded gene product, such as a desired biochemical or biological activity, for example binding activity.
Phage display technology has been highly successful as providing a vehicle that allows for the selection of a displayed protein by providing the essential link between nucleic acid and the activity of the encoded gene product (Smith, 1985; Bass et al., 1990; McCafferty et al., 1990; for review see Clackson and Wells, 1994). Filamentous phage particles act as genetic display packages with proteins on the outside and the genetic elements, which encode them on the inside. The tight linkage between nucleic acid and the activity of the encoded gene product is a result of the assembly of the phage within bacteria. As individual bacteria are rarely multiply infected, in most cases all the phage produced from an individual bacterium will carry the same nucleotide sequence and display the same protein.
However, phage display relies upon the creation of nucleic acid libraries in vivo in bacteria. Thus, the practical limitation on library size allowed by phage display technology is of the order of 107 to 1011, even taking advantage of λ phage vectors with excisable filamentous phage replicons. The technique has mainly been applied to selection of molecules with binding activity. A small number of proteins with catalytic activity have also been isolated using this technique, however, in no case was selection directly for the desired catalytic activity, but either for binding to a transition-state analogue (Widersten and Mannervik, 1995) or reaction with a suicide inhibitor (Soumillion et al., 1994; Janda et al., 1997).
Another method is called Plasmid Display in which fusion proteins are expressed and folded within the E. coli cytoplasm and the phenotype-genotype linkage is created by the fusion proteins binding in vivo to DNA sequences on the encoding plasmids whilst still compartmentalised from other members of the library. In vitro set on from a protein library can then be performed and the plasmid DNA encoding the proteins can be recovered for re-transformation prior to characterisation or further selection. Specific peptide ligands have been selected for binding to receptors by affinity selection using large libraries of peptides linked to the C terminus of the lac repressor Lacl (Cull et al., 1992). When expressed in E. coli the repressor protein physically links the ligand to the encoding plasmid by binding to a lac operator sequence on the plasmid. Speight et al. (2001) describe a Plasmid Display method in which a nuclear factor κB p50 homodimer is used as a DNA binding protein which binds to a target κB site in the −10 region of a lac promoter. The protein-DNA complexes that are formed have improved stability and specificity.
An entirely in vitro polysome display system has also been reported (Mattheakis et al., 1994) in which nascent peptides are physically attached via the ribosome to the RNA which encodes them.
In vitro RNA selection and evolution (Ellington and Szostak, 1990), sometimes referred to as SELEX (systematic evolution of ligands by exponential enrichment) (Tuerk and Gold, 1990) allows for selection for both binding and chemical activity, but only for nucleic acids. When selection is for binding, a pool of nucleic acids is incubated with immobilised substrate Non-binders are washed away, then the binders are released, amplified and the whole process is repeated in iterative steps to enrich for better binding sequences. This method can also be adapted to allow isolation of catalytic RNA and DNA (Green and Szostak, 1992; for reviews see Chapman and Szostak, 1994; Joyce, 1994; Gold et al., 1995; Moore, 1995).
WO99/02671 describes an in vitro sorting method for isolating one or more genetic elements encoding a gene product having a desired activity, comprising compartmentalising genetic elements into microcapsules; expressing the genetic elements to produce their respective gene products within the microcapsules; and sorting the genetic elements which produce the gene product having the desired activity. The invention enables the in vitro evolution of nucleic acids by repeated mutagenesis and iterative applications of the method of the invention.
In contrast to other methods WO99/02671 describes a man-made “evolution” system which can evolve both nucleic acids and proteins to effect the full range of biochemical and biological activities (for example, binding, catalytic and regulatory activities) and that can combine several processes leading to a desired product or activity.
A prerequisite for in vitro selection from large libraries of proteins is the ability to identify those members of the library with the desired activity (eg. specificity). However, direct analysis of the selected protein requires much larger amounts of materials than are typically recovered in such experiments. One way in which this problem can be addressed involves the creation of a physical association between the encoding gene and the protein throughout the selection process and so the protein can be amplified and characterised by the encoding DNA or RNA.
The present invention seeks to provide an improved method for the in vitro selection of polypeptide domains according to their binding activity.